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    <script>
        console.log(!!null);//false
        console.log(!!"");  //false
        console.log(!!1);//true
        // null是假值，!null返回true,   !true返回false
        //""是假值，!""返回true,   !true返回false
        //1是真值，!1返回false,   !false返回true
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